Method of making electrode systems



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Filed July Y5, 15539 @www a. nu

Ey e@ Maat@ Patented Dec. 2, 1941 METHOD F MAKING ELECTBQDE SYSTEMS Willem Christiaan van Geel, Eindhoven, Netherlands, assigner. by mesne assignments, to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application July 5, 1939, serial No. 232,95

In Germany July 29, 1938 3Claims.

My invention relates to a method of making an electrode system of unsymmetrlc conductivity which commises a selenium electrode.

In making an electrode it is the usual practice to spread out the selenium in its liquid state on a support, and it has been suggested to apply a reasonable pressure to obtain a transition into the gray conductive modification of the selenium. However, these two operations are usually can'led out separately and the selenium, after it has been applied to the support and become solid, is placed in a press, during which operation the temperature is increased. As a result the pressing is accompanied by an increase in temperature which has the effect that the increase in conductivity inherent in the transition into the grey modiiication is obtained.

The main object of my invention is to provide an improved method by which not only the specic resistance of the selenium acquires an advantageous value, but also the electrical properties are maintained constant to a higher degree than was possible with the known method.

In accordance with the invention the support with the selenium applied thereto is lirst heated to a temperature slightly below the melting point of selenium and subsequently the aggregate is cooled down while under pressure.

Electrode systems made by. the method of the invention are less subject to the so-called aging phenomenon than are the electrode systems manufactured by the known method. The term "aging phenomenon is to be understood to mean the always growing decrease in forward current with constant voltage as the electrode system becomes older.` This phenomenon has been recognized by K. Maier in Trockengleichrichter 1937, page 193, which states that blocking layer rectifiers exhibit a continuous decrease in forward current also in continuous operation. I shall hereinafter refer more fully to this phenomenon which also occurs in intermittent operation.

According to one embodiment of the invention the selenium is cooled during pressing from a temperature slightly below its melting point to a temperature between about 180 C. and 140 C., preferably about 160 C., and subsequently the electrode system is cooled down to room temperature without the use of pressure.

By pressing is meant a treatment during which the entire surface of the selenium is simultaneously subjected to a pressure which is constant and equal throughout the surface.

1 are probably due to the facts that because of the pressing, the various selenium particles are more closely pressed together and that, because of the decrease in temperature, these particles at the same time adhere In contradistinction to this, in the prior art method, the pressing is effected with an increase in temperature in which case the pressing together of the particles is less active because they are more'or less liquid and mixed up in an uncontrollable manner.

In order that the invention may be clearly understood and readily carried into eiect l shall describe the same in some detail with reference to a specific example and to the accompanying drawing` in which the single figure is a graph showing the decrease with age of the current o1' a rectifier with respect to the original current.

In accordance with the invention selenium is brought into its liquid state, and about 0.5% by weight of columbium .chloride (CbCl5) is added thereto while constantly stirring the mixture to obtain as ne a division as possible. A quantity of this selenium mixture is applied to the surface of a suitable support, `for instance a4 plate oi aluminum, which, if desired, may previously be Toughened, zinc-plated and provided with a graphite coating, as described in the U. S. patent application S. N. 210,796 led May 28. 1938. The selenium is preferably applied by centrifuging, because this method allows the selenium to be applied in the form of a very thin and uniform coating, for instance, about 0.2 mm. thick. According to this method of applying the selenium. which has been describedin detail in the U. S. patent application S. N. 254,508

filed February 3, 1939, a quantity of the selenium mixture is placed in the center of a plate which is rotated at high speed. During this operation the selenium must be maintained at a temperature of about 250 to 300 C.

The aluminum plate provided in the above manner with the selenium coating is then placed in an oven after the upper side o f the selenium coating has been covered with a metalor mica plate which is smeared on the selenium side with quinoline (CoH'zN) or another material which produces an alkaline reaction and has a high boiling point. The aggregate is then heated for several minutes in the oven at a temperature of about 190 to 210 C., preferably at about 200 C., and is then pressed for about 5 minutes at a pressure of about kgs. per sq. cm. in a press comprising two bodies having plane surfaces so that the pressure is equally distributed over the The advantages obtained by using my method 5o selenium surface. The temperature of the pres is maintained at about 155 C., so that during pressing the selenium is subject to a temperature drop of about 45 C.

'I'he quinoline acts upon the selenium during the heating to about 200 C. in the oven so that at that time the formation of the blocking layer is begun. While the layer is in the press, the blocking layer is strengthened and is given the thickness suitable for normal operation.

If it is desired to further increase the conductivity of the selenium, the selenium may remain in the press and at a temperature of 155 C., until the desired conductivity has been attained. subjected to a renewed subsequent treatment in the oven at a temperature of approximately 200 C. without applying pressure.

In neither of the above two casesA is the adhesion of the selenium particles disturbed by the pressing with a decrease in temperature. because the selenium does not become fluid again.

After the selenium electrode with the blocking layer has been manufactured, the conductive electrode still has to be applied to the blocking layer. This may be effected in known manner, for example, by spraying a low-melting alloy upon the blocking layerby means of a spraying gun. For this purpose I may use, for example, an alloy melting at about 103 C. and consisting of tin, bismuth and cadmium in a proportion by weight of 26:53:21.

The advantage of the invention as regards the diminution of the aging phenomenon with respect to known rectiers will be explained more fully with reference to the logarithmic graph in which the abscissae represent the age of the rectifler in days and the ordinates represent the decrease in current in amps., with respect to the original current. It will be noted that the abscissae start at one day of aging.

In the graph the curves a and b represent the results obtained with two rectiflers a and b of equal dimensions. The two rectiers were manufactured in the same manner except that rectifier a was pressed with an increase in temperature and the rectiier J was, in accordance with the present invention, lpressed with a decrease in temperature. The two rectifiers immediately after manufacture exhibited a forward current of L1.0 amps. when a direct voltage of 2 volts was applied. After being stored for about twenty-four hours, the two rectiers were measured again in the same manner. From the curves it appears that after one day of aging rectiiier a shows a decrease in current of 1.4 amp., i. e. transmits a forward current of 2.6 amp., whereas rectifier b shows a decrease in current of 1.5 amp. i. e. a forward current of 2.5 amp. Periodic measurements show that the decrease in current of the rectifier a becomes greater and greater' with an increase of age, whereas the decrease in current of the rectifier b, which initially may be greater than that of rectifier a, immediately becomes lower.

Instead of this, the aggregate may be From a comparison of the curves a and b it appears that after about 1.3 days of aging, a total aging-of the original arrear of the rectifier b with respect to a is recovered. 'I'he forward current of a decreases more and more and after about 3 months (90 days) the outgoing current is about 1.8 amp. and has consequently fallen down to a value of less than half the original outgoing current (4.0 amp.). This decrease continues, though less rapidly.

With the rectier b, however, the'decrease in current gradually becomes smaller and after 3 months the outgoing current has risen again up to 2.85 amp., i'. e.' to about 70% of the original value. A gain of about 50% with respect to the rectifier a has been obtained with the same aga in which case the fact must be considered that the curve b shows a rising tendency, i. e. that the ratio becomes more and more favorable.

While I have described my invention with reference to specific examples I do not wish to be limited thereto because obvious modiiications in my method will present themselves to one skilleclln the art.

What I claim is:

1. In the manufacturing of an electrode system of unsymmetric conductivity, the steps of applying a coating of selenium to a support. heating the coated support to a temperature above about 190 C. and below the melting point of the selenium, cooling the coated support to a temperature of about C. while applying pressure to the selenium coating, and cooling the coated support without applying pressure to the selenium coating.

2. In the manufacture of an electrode system of unsymmetric conductivity, the steps of applying a coating of selenium to a support, applying to the surface of the selenium coating a material adapted to produce an alkaline reaction, heating the coated support to a temperature above C. and below the melting point of the selenium to form a blocking layer by the action of said material on the selenium coating, cooling the coated support from said temperature while applying pressure to the selenium coating, and cooling the coated support to room temperature in the absence of pressure.

3. In the manufacture of an electrode system of unsymmetric conductivity, the steps of applying a coating of selenium to a support, heating the coated support to a temperature above about 190 C. and below the melting point of the selenium to form a blocking layer by the action of said material on the selenium coating, cooling the coated support to about 140 C. to 180 C. while applying pressure to the selenium coating, maintaining the selenium coating at said latter temperature to adjust the conductivity thereof, and cooling the coated support to room temperature in the absence of pressure.

WILLEM CHRISTIAAN VAN GEEL. 

